In an aldol condensation reaction, an aldehyde or ketone, one of which must have a hydrogen atom alpha to the carbonyl, react to form a β-hydroxy aldehyde or a β-hydroxy ketone (hereinafter collectively referred to as “β-hydroxy carbonyls”). A principal benefit of the aldol reaction is that it forms new carbon-carbon bonds. The initial β-hydroxy carbonyl product can react further (in the presence of an acid or a base) to yield an α,β-unsaturated aldehyde or ketone (hereinafter collectively referred to as “α,β-unsaturated carbonyls”). A generic aldol reaction scheme appears as follows:

When two different aldehydes or ketones are reacted, and both reactants have an alpha-position hydrogen, four aldol products are possible:

As shown in this general scheme, R1 through R6 are each independently selected from the group consisting of hydrogen, hydroxy, C1-C8 alkyl, alkenyl, and cycloalkyl, C1-C10 mono- and bicyclic aromatic and heterocyclic moieties (including heterocyclic groups derived from biomass), and carbonyls and carbohydrates such as ethanedione, glyceraldehyde, dihydroxyacetone, aldotetroses, aldopentoses, aldohexoses, ketotetroses, ketopentoses, ketohexoses, and the like (without limitation).
However, when one of the carbonyl reactants lacks an alpha-position hydrogen, or cannot form an enolate, or otherwise has a relatively unreactive carbonyl group, the resulting reaction (commonly referred to as a “crossed” aldol reaction) yields a major product, usually in good yield. The mechanism is conventionally considered to be a nucleophilic addition of an enolate ion onto the carbonyl group of another, un-ionized reactant. The aldol reaction is generally quite selective, with yields greater than 80%.
The scientific literature describes a host of variations on the basic aldol condensation mechanism shown above. See, for example, Published U.S. Pat. Appl. 2005/0,004,401; U.S. Pat. No. 5,583,263; U.S. Pat. No. 5,840,992; U.S. Pat. No. 5,300,654; Kyrides (1933) J. Amer. Chem. Soc. 55:3431-3435; and Powell (1924) J. Amer. Chem. Soc. 46:2514-17.
Published PCT Appl. WO 00/00456 describes performing aldol condensation reactions using a base-modified clay as a catalyst. The resulting aldols may be reacted further via hydrogenation to yield the corresponding 1,3-diols. Similarly, Published U.S. Patent Appl. 2004/0,138,510 describes co-producing unsaturated aldehydes via a crossed-aldol condensation catalyzed by a water-soluble phase-transfer catalyst. The resulting aldols may be further reacted to yield desired alcohol products or saturated aldehyde feedstocks. Published PCT Appl. WO 01/02330 describes an aldol reaction between an aldehyde and formaldehyde (i.e., a crossed-Cannizzaro reaction), followed by hydrogenation of the aldol product to yield polyols having three or four hydroxyl groups.
Japanese Patent JP 62 192 335 describes a process for making diacetone alcohol. The process includes subjecting acetone to an aldol condensation in the presence of magnesium oxide containing a metallic catalyst selected from sodium, copper, zinc, zirconium, manganese, iron, nickel or chromium.
Aqueous-phase aldol condensation reactions have previously been carried out with glyceraldehyde, dihydroxyacetone, formaldehyde and butyraldehyde using both homogeneous and heterogeneous base catalysts. See Gutsche et al. (1967) J. Amer. Chem. Soc. 89:1235, and Serr-Holm et al. (2000) Appl. Catal. A 198:207. Cross condensation of furfural with acetone has been conducted using amino-functionalized mesoporous base catalysts, Choudary et al. (1999) J. Mol. Catal. A 142:361. Mixed Mg—Al-oxides have previously been used as solid base catalysts for liquid-phase aldol condensation reactions. See Sasaki, Goto, Tajima, Adschiri & Arai (2002) Green Chem. 4:285, and Climent, Corma, Iborra, Epping, & Velty (2004) J. Catal. 225:316 (2004).
A host of other types of catalytic systems for carrying out aldol and other carbon-carbon bond-forming reactions have been described in the scientific literature. See Serra-Holm et al. (2000) Applied Catalysis A: General 198:207-221 (anion exchange resin catalyst); Cordova et al. (2002) Chem. Commun. 3024-3025 (cyclic secondary amine catalyst); Aramendia et al. (2004) J. Mol. Catalysis A: Chemical 218:81-90 and Aramendia et al. (2004) Colloids & Surfaces A: Physicochem: Eng. Aspects 234:17-25 (magnesium- and magnesia-containing catalysts); Climent et al. (2004) J. Catalysis 221:474-482 (activated hydrotalcite catalyst in a Claisen-Schmidt condensation); and Roelefs et al. (2001) Catalysis Letters 74(1-2):91-94:
In the face of natural disasters (principally hurricanes impacting the gulf coast of the United States) and political instability in the oil-producing countries of the world, the production of liquid fuels from renewable biomass resources is becoming increasingly more attractive. This attractiveness is further heightened as gasoline- and diesel-powered hybrid electric vehicles, having overall energy efficiencies comparable to vehicles powered by fuel cells, are being sold commercially. For example, see Weiss, Heywood, Schafer & Natarajan, “Comparative Assessment of Fuel Cell Cars,” No. 001, MIT Laboratory for Energy and the Environment, © 2003. Moreover, many industrialized and industrializing countries, including the United States, grant significant tax incentives for producing liquid bio-diesel for use as transportation fuel. See, for example, U.S. Internal Revenue Service Circular 378, cat. no. 46455F (April 2005).
Approximately 75% of the dry weight of herbaceous and woody biomass is comprised of carbohydrates. See Klass, “Biomass for Renewable Energy, Fuels and Chemicals,” Academic Press, San Diego, © 1998. Several processes currently exist to convert carbohydrates to liquid fuels, including forming bio-oils by liquefying or pyrolyzing biomass (Elliott et al. (1991) Energy and Fuels 5:399.), producing alkanes or methanol by Fischer-Tropsch synthesis from biomass-derived CO:H2 gas mixtures (Klass, supra), and converting sugars and methanol to aromatic hydrocarbons over zeolites catalysts (see Chen, Degnan & Koenig (1986) Chemtech 16:506; and Weisz, Haag & Rodewald (1979) Science 206:57).
Currently, however, converting glucose to ethanol is the most widely practiced process for producing liquid fuels from biomass. Katzen & Tsao (2000) Adv. Biochem. Eng/Biotechnol 70:77. The overall energy efficiency starting from corn (i.e., the heating value of the product ethanol divided by the energy required to produce ethanol from corn) is about 1.1 without accounting for co-product energy credits. See Shapouri, Duffield & Wang, “The Energy Balance of Corn: An Update,” No. 814, U.S. Department of Agriculture, Office of the Chief Economist, © 2002. An astonishing 67% of the energy required to produce ethanol from corn is consumed in the fermentation/distillation process. Of that 67%, over half of the energy is used to distill ethanol from water. See Shapouri et al., supra, and Katzen et al., in “Fuels from Biomass and Wastes,” Klass & Emert, Eds., Ann Arbor Science, Ann Arbor, © 1981, pp. 393-402.
In comparison, a practical route to produce long-chain alkanes from an aqueous carbohydrate solution would not require an energy-intensive distillation step because the product long-chain alkanes would spontaneously separate from aqueous solvent. Again using the values provided by Shapouri et al. (supra), it is estimated that the overall energy efficiency for producing alkanes from corn would rise to about 2.2 if the production process did not require a final distillation step. This estimate is underpinned by several well-founded assumptions, namely: (1) that the production process still requires all of the remaining energy needed to produce ethanol from corn; (2) that the yields for sugar and ethanol production are as reported by Klass (supra); and (3) that sugars are converted into alkanes as given by a stoichiometry analogous to Eq. 3, below. (See the Examples for a fully detailed set of calculations.) In short, all other considerations being equal, if the conventional distillation step can be omitted, the overall energy efficiency of producing liquid alkanes from corn can be doubled as compared to conventional techniques requiring fermentation/distillation.
It has recently been shown that an aqueous solution of sorbitol (the sugar-alcohol of glucose) can be converted to hexane (Eq. 1) with a catalyst containing both acid sites (e.g., SiO2—Al2O3) and metal sites (e.g., Pt or Pd) to catalyze dehydration and hydrogenation reactions, respectively. Huber, Cortright & Dumesic (2004) Angew. Chem. Int. Ed 43:1549. Hydrogen for this reaction can be produced from aqueous-phase reforming of sorbitol (Eq. 2) in the same reactor or in a separate reactor with a non-precious metal catalyst. Huber, Shabaker & Dumesic (2003) Science 300:2075. The net reaction (Eq. 3) is an exothermic process in which approximately 1.5 moles of sorbitol produce 1 mole of hexane.C6O6H14+6H2→C6H14+6H2O  (1)C6O6H14+6H2O→6CO2+13H2  (2)

Alkanes produced in the aqueous-phase dehydration/hydrogenation (APD/H) of carbohydrates could provide a renewable source of transportation fuel to complement the rapidly growing production of bio-diesel from vegetable oils and animal fats. Ma & Hanna (1999) Bioresour. Technol. 70:1. Unfortunately, the high volatility of hexane makes it of low value as a fuel additive. Owen & Coley, “Automotive Fuels Handbook,” Society of Automotive Engineers, Warrendale, Pa., © 1990. Thus, there remains a long-felt and unmet need for a practical and energy-efficient process for producing high-quality, long-chain liquid fuels (e.g. C6 to C15 alkanes) from carbohydrates.